Typically, the progression of a disease in the biological tissue is accompanied by changes in tissue mechanical properties. Therefore, the availability of the modality effectuating the measurement of the mechanical properties of tissue in situ, in its native state, would provide critical diagnostic information.
Laser Speckle Rheology (LSR) is an optical approach that enables non-contact probing of tissue viscoelasticity. In LSR, tissue is illuminated by a mono-chromatic laser source and a high-speed CMOS camera is used to capture temporal intensity fluctuations of back-scattered laser speckle patterns. The temporal speckle intensity fluctuations are exquisitely sensitive to the displacements of light scattering centers undergoing Brownian motion, and the extent of this thermal motion reflects the viscoelastic properties of the surrounding medium. Speckle frames acquired by the high-speed camera are analyzed to obtain the speckle intensity temporal autocorrelation curve, g2(t). The g2(t) curve is a measure of the rate of temporal speckle intensity fluctuations and is closely related to the extent and time scales of particle motion, and in turn mechanical properties of the medium, defined by the viscoelastic modulus, G*(ω)=G′(ω)+iG″(ω), which defines the mechanical behavior of materials. Traditionally, G*(ω) is measured using a mechanical rheometer, by evaluating the ratio of an applied oscillatory stress to the corresponding induced strain in the specimen, over a limited oscillation frequency range. Using the LSR, the viscoelastic modulus, G*(ω), can be assessed in a non-contact manner, by analyzing the g2(t) and retrieving displacement of scattering particles, often quantified by the mean square displacement (MSD), denoted as <Δr2(t)>. The generalized Stokes-Einstein relation (GSER) is then used to extract the viscoelastic modulus, G*(ω), from the measured MSD. For relatively soft materials, the Brownian movements of scattering particles are fast and MSD increases quickly with time, eliciting rapid speckle fluctuations. In contrast, for mechanically rigid materials, scattering particles exhibit confined motions around a fixed position, which lead to restrained growth of MSD and slow variation of speckle patterns.
The primary challenge in extracting the viscoelastic modulus of tissue from speckle frame series lies in assessing the MSD from the measured g2(t) curve(s), partly because the rate of temporal speckle fluctuations depends not only on the Brownian displacement of scattering centers but also on the optical properties of the tissue—such as absorption and scattering coefficients and scattering anisotropy factor (μa, μs, and g)—that determine the transport of light within the illuminated volume. Accordingly, in order to accurately measure sample mechanical properties using LSR, it is required to isolate the influence of optical absorption and scattering from the g2(t) measurements to accurately describe the MSD.
Traditionally, diffusing wave spectroscopy (DWS) formalism is used to describe the relationship between the measured g2(t) and MSD for strongly scattering media with negligible absorption. In such media, light transport is often assumed to be diffusive. The majority of biological fluids and tissue, however, exhibit considerable absorption (μa>0), and highly anisotropic scattering (g˜0.9), and back-scatter light rays with sub-diffusive characteristics. In this case, the simple DWS formalism is modified to incorporate the knowledge of optical properties of the tissue to better explain the relationship between g2(t) and MSD. To compensate for shortcomings of the DWS, which assumes diffusive light transport, an analytical solution termed the “telegrapher equation” has been proposed. The telegrapher approach shares the ease and simplicity of the DWS expression but aims to incorporate the attributes of strong absorption, scattering anisotropy, and non-diffusive propagation of rays within short source-detector distances in a modified photon-transport equation. Alternatively, a Monte-Carlo ray tracing (MCRT) algorithm may be used to simulate the propagation of light in a medium of known optical properties and derive a numerical solution for speckle intensity temporal autocorrelation curve as a function of particle Brownian displacement.
A new polarization-sensitive correlation transfer (PSCT)-MCRT algorithm was proposed for describing light propagation in purely scattering media and accounting for the fluctuations of scattered light (See Z. Hajjarian and S. K. Nadkami, “Evaluation and Correction for Optical Scattering Variations in Laser Speckle Rheology of Biological Fluids,” PLoS ONE 8, e65014, 2013). The performance of PSCT-MCRT in estimating the MSD of Brownian particles in purely scattering media showed improved accuracy of estimating sample mechanical properties compared to the DWS approach [3]. Most biological tissues, however, in addition have light absorbing characteristics that are typically not taken into account in devising the MSD.
Fluid Biological Tissues—biological fluids (also referred herein as biofluids)—such as cerebrospinal fluid (CSF) mucus, synovial fluid, and vitreous humorous function as shock-absorbents, allergen and bacteria trappers, and lubricants in different organs and organ systems. Biofluids have distinct rheological characteristics and exhibit both solid-like and fluid-like behavior over different loading conditions and size scales. As the evidence of correlation between viscoelastic properties of biofluids and initiation and progression of various bodily maladies becomes available, there arises a need in development of a methodology that would allow the user to evaluate mechanical properties of biological fluids in situ under native conditions to advance clinical disease diagnosis and treatment monitoring.